The present invention relates to electroplating metals. In general, metals are difficult to electroplate to a resistive substrate, such as a conductive plastic, a carbon-filled plastic, an insulator with a resistive coating, and a resistive porous electrode. Because of the resistivity of the substrate, metal ions in the electroplating bath are deposited around electrical contact points of the resistive substrate, causing a non-uniform metal layer to develop on the substrate. In particular, a large accumulation of the metal will occur around the electrical contact points, and a negligible amount of metal will accumulate elsewhere.
For example, as shown in the cross-sectional view of FIG. 1, to electroplate a copper layer 1 onto a silicon wafer 2, a barrier film 3 having resistive properties is first placed on the silicon wafer 2. Conventionally, when copper is electroplated onto the barrier film, copper is deposited around the electrical contact points, which are usually located on the perimeter of the silicon wafer. This resulting copper layer is non-uniform, as illustrated in FIG. 1. In particular, the copper is deposited in an upwardly extending rim around the perimeter of the barrier film 3, and negligible amounts of copper are deposited in the middle of the barrier film 3.
To correct for this non-uniform layer, an initial copper layer is deposited on the barrier film through a means other than electroplating. For example, sputtering, chemical vapor deposition or electroless plating is conventionally used to deposit the initial copper layer on the barrier film. This results in forming a layer with relatively low resistance, and which is conducive to being electroplated with copper. However, this conventional approach has drawbacks. In particular, the sputtering and chemical vapor deposition are expensive. Moreover, electroless plating is slow and is difficult to control. Thus, improved methods for electroplating metals, particularly copper, onto a resistive substrate are desired.